blower motor mystery

lakerd Posted By lakerd, Jul 15, 2019 at 4:08 PM

  1. lakerd

    lakerd
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    Apr 21, 2018
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    I have a shaded pole blower motor with speed control that doesn't maintain constant speed beyond a certain setting. With the speed control plugged in to 115v and no load I can measure the voltage output varying constantly from 75v to 90v. When I plug two spare blowers into the controller the speed can be varied constantly just as expected. But when I plug my fireplace blower in and turn it up halfway, I measure the voltage under load at 90v and it maintains constant speed for a few minutes, then it increases to almost max speed while the voltage remains constant. Then I can vary the voltage from 90 to 115 and back and the speed just stays the same. If I drop it way down to about 85v it will eventually slow down to the expected speed. But if I move it back up to 90v it will eventually ramp up to max speed again. Any insights as to what is going on here and if it can be fixed? Apparently it is a problem with the motor and not the controller.
     
  2. jetsam

    jetsam
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    It can be complicated.

    Back in the dusty past, you might have controlled a motor with a power rheostat (a big variable resistor). It couldn't be set too high or the resulting brownout would fry the motor. It also dumps off the difference between full power and its current setting as heat, since it is a big resistor. Not very useful and very inefficient.

    These days, the cheapest way to control AC motor speed is through VFD- basically you convert the AC to DC, then use the DC to make a new pulsed AC waveform that has variations in voltage and frequency.

    I am guessing (hoping?) that blowers have gone VFD over the years. It's cheap these days. If you have a VFD control, you cannot measure the output with a multimeter- though you can see it on an oscilliscope or frequency analyzer.
     
  3. EatenByLimestone

    EatenByLimestone
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    If a thermostat is used to control a blower on a heating appliance, is the excess heat still considered an inefficency?
     
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  4. Ludlow

    Ludlow
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    Witty! But actually the heat is just wasted electricity. And the little heat generated costs real money for what is produced.
     
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  5. jetsam

    jetsam
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    Efficiency is useful work performed for energy used.

    So if you put a window air conditioner in the middle of the room and ran it, it would be 0% efficient if you wanted it to be colder in the room, or close to 100% efficient if you wanted it to be hotter in the room.

    If it was in a room with two people, one of which wanted it colder and one of which wanted it hotter, I am not sure how you'd figure it. 50% is the compromise number, and that number is demonstrably wrong. :)

    In your example with a blower on a rheostat, it's still partly a psychology question. Does the user consider his wood-fired BTUs or his electrical BTUs cheaper? (It almost doesn't matter though, because for most stoves, the blower is using tens of joules of electricity to harvest hundreds of joules of heat that would otherwise go up the flue.... until the fire goes out.)

    You can reduce it to a math question by using a common unit like joules on both sides, but your user might not agree with the answer.
     
  6. EatenByLimestone

    EatenByLimestone
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    I've always wondered about stuff like that.


    At my cabin I keep a 100w bulb in a corner lamp. When I enter in the winter, I flip the light on. I always figure the light/heat ratio works in my favor.
     
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  7. jetsam

    jetsam
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    Those things being space heaters is why MOST of us got rid of 'em 30+ years ago.... ;)
     
  8. ckr74

    ckr74
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  9. lakerd

    lakerd
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    Apr 21, 2018
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    Tried my neighbor's controller and same thing happened. I'm thinking if the fan speed increases but voltage and I assume frequency remain constant, then the current is increasing somehow. Don't think it's temperature related. Something to do with a capacitor or resistor I guess. I don't know enough about that stuff or what is in my controller. I can't even find a way to get it open without destroying it.
     

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